U.S. application Ser. Nos. 12/807,792 and 12/807,793 (the “'792 and '793 applications”), incorporated herein by reference in their entireties, disclose gold/gold sulfide (“GGS”) nanoparticles within a chitosan matrix, and methods for synthesizing and using the same. These references disclose forming GGS nanoparticles by self-assembly of a sulfide source, such as sodium thiosulfate, and a gold source, such as cholorauric acid. GGS nanoparticles have absorbance peaks in the near infra-red (“NIR”) region, which are tunable by varying the self-assembly conditions. During self-assembly of GGS nanoparticles, gold colloid is simultaneously formed as a byproduct. In the '792 and '793 applications, gold colloid particles were removed using one, or more commonly three, rounds of centrifugation.